Genus: Highly flattened; pellicle firm; body form constant (Kudo, 1966). Chloroplasts small, discoid, pyrenoids usually absent; most species flat and leaf-shaped; often with ridges or fins running helically or longitudinally (Illustrated Guide, 1985).
Species:
Similar species -->> P. acuminatus;
P. circulatus
47 x 37 μm
Myoyo
Fukiage
Saitama, 2004
P. circulatus ?
55 x 38 μm
near Yamaguchi Univ.
Yamaguchi, 2004
P. circulatus ?
45 x 33 μm
Onnenai
Kushiro marsh
Hokkaido, 2005
P. circulatus ?
40,43 x μm
Nozomiko 2
Iiyama
Nagano, 2007